The present invention relates to magnetic bubble memories, and more particularly to methods for routing of data to and from only non-defective minor loops in bubble memories employing redundant minor loops.
Currently, the most popular architecture for magnetic bubble memories is that in which magnetic bubbles are stored in a plurality of parallel minor loops. In a block replicate bubble memory a serial-parallel bubble propagation path is used to transfer bubbles into parallel minor loops, and a parallel-serial bubble propagation path is used to transfer bubbles from the minor loops.
However, due to a variety of causes, some of the minor loops develop defects during their fabrication. For example, the defects may be due to flaws in the garnet film, which resist bubble propagation. They may also be due to permalloy shorts attributable to photolithographic tolerances, or due to dust and other impurities which enter the memory during its fabrication. As a result, some small percentage of the minor loops, such as five percent or less, typically do not work.
To overcome this problem bubble memories are fabricated with extra or so-called redundant minor loops. Then all defective loops, and any good loops in excess of a design number simply are not used during the write and read operations. Suppose for example, that a bubble memory chip was designed to have 256 fault free minor loops. To achieve this the memory would be built with some larger number of minor loops, such as 270. Then in most cases, at least 256 loops would be fault free.
The problem then becomes one of devising a method for bypassing the defective loops as information is written into and read from the memory. Suppose that in the preceding example, loops ten and one hundred were defective. Under those conditions, no data could be written into the memory on the tenth and one hundredth clocking pulses out of every 270 clocking pulses. Similarly, during a reading operation, the tenth and one hundredth information bits out of every 270 bits of information that are read from the memory must be ignored.
In the past such control functions have been performed with the aid of external control circuitry. One common prior art control circuitry includes a read only memory (ROM) that is programmed to indicate which of the loops are defective. Address counters simultaneously address both the ROM and the bubble memory. When the ROM output indicates that the bubble memory location being addressed is defective, corrective action is taken. For a discussion of the ROM approach to mask out defective minor loops see the book entitled Magnetic-Bubble Memory Technology written by Hsu Chang, pages 35-36, copyright 1978, and published by Marcel Dekker, Inc.
More recently, some bubble memories have included a single extra loop on the chip for storing error map information therein. The user is thus able to read the error map from the memory during system initialization and the error map is thereafter stored in a random access memory (RAM). The RAM is then used in conjunction with control logic to mask out the defective loops during a write or read operation. Published German patent application No. 2804695 filed by Texas Instruments, Inc. is believed to be illustrative of this last mentioned approach.
Both of the above approaches, however, have certain deficiencies. For example, with the ROM approach, each memory system requires unique parts. That is, the ROM for one memory system cannot be used as the ROM in another memory system because the bubble memory chips in each memory system have different defective minor loops. Furthermore, when a bubble chip in a memory system goes completely bad, due to aging for example, the replacement of the bubble chip also neccessitates the replacement of the ROM.
One problem with the prior are bubble memory chips that provide an on chip error map loop is that they include no redundant error map loops. That is, they include only a single error map loop. If that loop is defective, then the entire chip must be discarded. Thus, the production yield of those bubble memories chips is undesirably low. Furthermore, there is a risk when storing the error map in an extra minor loop that the starting and stopping of the drive field will not be precise with regard to 360.degree. of orientation. Because of this, it is possible for magnetic bubbles of the error map to jump between adjacent permalloy propagation elements when they are not supposed to. This so-called data scrambling results in the loss of the error map.
U.S. Pat. No. 4,228,522 assigned to the assignee of the present application, discloses a bubble memory that includes a plurality of minor loops for storing bubbles representative of data therein, and a pair of minor loops for storing bubbles representative of an error map therein. Proper choice of the number of propagation elements enables the error map to be selectively written into and read from only one loop of the pair of error map loops with only a single control line. This design produces the high chip yield.
Other patents of interest which deal with the problem of masking out defective minor loops in a magnetic bubble memory are U.S. Pat. Nos. 3,909,810; 4,073,012; and 4,090,251.
One problem with the error map approach to handling defective minor loops is that the added control circuitry increases the cost of the bubble memory system. Furthermore, it increases the complexity of the chip design. Also, frequently the bubble memory chips are not interchangeable. In addition, the operating speed of a bubble memory incorporating the error map approach is slower than it would be if the defects were transparent to the user. Consider again the above described example that had 270 total loops of which only 256 were guaranteed to be non-defective. In that memory, a total of 270 revolutions of an external magnetic drive field are required to load one bubble into each of 256 minor loops.
Conversely, if the defective loops were transparent to the user, a total of only 256 rotations of the magnetic field would be required to load one bubble into each of the loops.
In U.S. Pat. No. 4,233,670, assigned to the assignee of the present application, there is described a fault transparent magnetic bubble memory in which magnetic bubbles are routed away from defective minor loops by selectively destroying certain permalloy shorting bars to convert double-distance bubble propagation elements to single-distance bubble propagation elements. This is done in both the serial-parallel input and parallel-serial output propagation paths. As a result, the number of rotations of the magnetic drive field that are required to load one bubble into each of 256 non-defective minor loops and to read one bubble from each of these loops can be reduced to 256. The defective minor loops are determined during the fabrication process and preselected ones of the permalloy elements are destroyed using a computer controlled laser beam. The need for error map storage loops, control circuitry for masking out defective minor loops pursuant to the error map, and complex permalloy patterns for handling the on chip propagation and storage of the error map, is eliminated.
Aforementioned U.S. Pat. No. 4,233,670 describes two different techniques for destroying a selected permalloy element. In one method, a laser beam of sufficient power is applied to the element for sufficient duration so that the permalloy element is vaporized and effectively severed at one point. The other method is to heat the permalloy element with the laser beam to a temperature sufficient to substantially degrade the magnetic properties of the element. This will insure that the element acts as a barrier over which magnetic bubbles will not propagate.
Both of the above-described laser beam approaches to destroying selected permalloy elements in order to mask out the defective minor loops have certain drawbacks. It is difficult to clearly and reproduceably severe a permalloy propagation element on a bubble memory chip with a laser beam. Molten material and gas which are generated upon laser beam cutting, tend to escape violently. This can result in rupturing of overlying materials and contamination of adjacent areas with debris. Frequently, the thermal conductivity of the bubble chips varies from one location to the next. A laser blast that might neatly severe a permalloy element in one area of the chip may not accomplish the same objective with regard to an element in another area of the chip. The laser beam can be automatically driven over different elements via computer control. However, it is not possible to continuously vary the intensity of the beam in accordance with the different thermal conductivities of the chip at different locations.
It would be desirable to develop an improved method of heating preselected permalloy propagation elements on a bubble memory chip which would insure a uniform degradation of their magnetic properties so that they would each present barriers to bubble propagation. Such an improved method would preferrably utilize a computer driven laser beam which can be accurately applied to individual microelectronic permalloy elements. Such an improved method would have to permit a wider range of temperatures than the methods of Reyling described above. In one of the Reyling methods the permalloy elements are heated to a critical temperature below the melting point of the permalloy in order to degrade their magnetic properties to a sufficient degree. It would further be desirable that this improved method effectively destroy the magnetic properties of the permalloy elements to the same degree that would result from physical severing, but without the violent effects associated with physical severing by laser beam.
An example of the process which might accomplish the desired local loss of magnetization is oxidation. This method requires, as a source of oxygen, an insulating layer in contact with the permalloy which is less thermodynamically stable than the oxides of NiFe. The commonly used insulating layers of materials such as Al.sub.2 O.sub.3 and SiO.sub.2 are not satisfactory. Thermodynamically, the bond strengths of NiFe with oxygen are relatively weak and thus this approach would be structurally unsound. Furthermore, it is believed that the search for a suitable insulating layer material for this application would probably be futile.
Another approach would be to select a material which, upon diffusion into the permalloy elements, would tend to destroy their magnetization. This material could be deposited by any conventional technique, e.g. sputtering, vacuum deposition, or plating, on either side of the permalloy layer. Diffusion into the permalloy elements would take place locally by selective laser heating. The heating would be limited so that splattering and other physical disruption would be minimized or eliminated thus increasing the process latitude. The magnetization "poison" material could be photolithographically etched at the same time that the permalloy is etched to define the individual propagation elements.
It is known that the introduction of nonferromagnetic copper into permalloy tends to quench its magnetization. However, it would not be suitable to form a copper film on either side of the permalloy propagation elements in a magnetic bubble memory because of the high electrical conductivity of copper. This conductivity would tend to place a short circuit in parallel with the permalloy elements of the bubble detector, thus degrading the detector's sensitivity. For example, a film of permalloy having a thickness of approximately 4,000 angstroms would have a resistance of about 0.5 ohms/square. If a film of copper having a thickness of approximately 200 angstroms were to be formed on the just mentioned permalloy film, it would represent a five percent addition in thickness. This copper film would have a resistance of about 0.8 ohms/square.
It has also been reported that manganese tends to quench the magnetization in permalloy. See the article entitled "Properties of Manganese-Permalloy Films" written by A. J. Griest and B. L. Flur, and published in the Journal of Applied Physics, Volume 38, No. 3, March 1, 1967 pages 1431-1433. Manganese is an excellent choice as a magnetic quenching agent for permalloy propagation elements in a magnetic bubble memory because its resistivity is very high. For example, a film of manganese having a thickness of approximately 200 angstroms has a resistance of about 70 ohms/square. Adding this film to a film of permalloy having a thickness of approximately 4,000 angstroms would change the resistance of the bubble detector by only three percent. This is an acceptably low value. Thus, manganese, in relatively low concentrations, would achieve the objective. The calculations in the above examples concerning copper and manganese films are only approximate and are set forth merely for comparison. Bulk values of resistivity were used in the calculations.
Another article of interest is entitled "Toward a Single Mask Processing of Ion-implanted Bubble Devices" by K. Y. Ahn and S. M. Kane, published in the IEEE Transactions on Magnetics, Vol. MAG-15, 1979, pages 1648-1650. It discloses a fabrication process for a bubble memory in which the permalloy layer is not etched. Instead, manganese is selectively diffused into the permalloy layer to define individual propagation elements. A laser beam is not utilized.